This invention relates to a stereotactic radiosurgery for effecting medical treatment by concentrating stereotactically a narrow beam of a radiation on one point. More particularly, the present invention relates to a stereotactic radiosurgery which will be suitable for correctly concentrating a radiation on one stereotactic point for a defect of mechanical accuracy of a gantry for irradiating a radiation and for any focuses.
An irradiation method of a radiation referred to as a "stereotactic method" has drawn an increasing attention. It is a method of medical treatment which imparts a high dose of a radiation to only a focus from multiple directions and destroys the focus. According to this method, the dose of the radiation to normal tissues around the destroyed focus is extremely small, and any adverse effects on the normal tissues can be eliminated.
Stereotactic radiosurgeries include a multi-beam source system radiosurgery referred to as a "Gamma Unit" and a mono-beam source system radiosurgery (PCT Japan Publication No. Hei-2-503521 (W089/05171) entitled "Stereotactic Radiosurgery System SRS 200 Isocentric Subsystem", PHILLIPS).
The former comprises a hemispherical collimator having a large number of irradiation holes and a cobalt 60 sealed beam source situated outside this collimator, and can accomplish medical treatment by applying concentratedly a gamma dose from a large number of cobalt 60 beam sources situated hemispherically to a focus. The latter rotates a gantry with the focus of a patient as a center by the use of an electron beam accelerator, etc, and can concentratedly irradiate the radiation to the focus from all directions while rotating a treatment couch. Accordingly, both can apply a large dose as an accumulated dose to the focus, and can reduce the dose to the normal tissue by a dispersion effect of the dose.
However, the former uses a large number (about 200) of cobalt 60 as the beam source. Since cobalt 60 has half life, exchange becomes necessary, and problems of maintenance and management occur. Furthermore, since the cost of the apparatus is high, this system has not yet been wide spread. The Gamma Unit of this system is exclusively for the head, and is not suitable for the chest and the abdomen at which the focus moves with the respiration. Accordingly, stereotactic surgery using the electron beam accelerator according to the latter has been expected. Stereotactic radiosurgical treatment using the electron beam accelerator will be explained generally with reference to FIGS. 1 and 2. A gantry 102 of an electron beam accelerator rotates with a horizontal axis 123 being the center, and a beam passes through an irradiation head 113 mounted to the radiation gantry 102, and is narrowed and irradiated to a focus. A point of intersection 120 between the horizontal axis 123 and the beam center 121 irradiated by a collimator is an isocenter, and the focus of a patient is brought into conformity with this position. The beam center must always catch the point of intersection 120 even while the gantry 102 is rotating with the horizontal axis 123 being the center. The radiation can be irradiated to the focus from all directions by operating in a composite way the rotation of the gantry 102 and the rotation of a treatment couch 112 inside a horizontal plane.
The gantry and the irradiation head mounted to the gantry are very heavy. Therefore, if any shake or deflection occurs during the rotation of the gantry as indicated by dotted line in FIGS. 1 and 2, the beam center cannot catch the focus and the focus of the radiation beam does not pass through the isocenter as represented by reference numerals 120a and 120b. When the medical treatment is carried out under such a state, the ratio of the dose to the focus and the dose to the normal tissue becomes small, so that the destroying effect of the focus becomes also small and even the normal tissue might be destroyed. Moreover, followup performance of the irradiation head, which is a heavy device, with the chest and the abdomen moving with the respiration is low, and the system must be increased in scale to accomplish this operation.